<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>边框</title>
  <style type="text/css">
    div{
      width: 50%;
      height: 100px;
      border: 1px dashed blue;
    }
  </style>
</head>
<body>
<div>小勤学CSS</div>
</body>
</html>